Heretofore, as the most common printed circuit board, a board has been used wherein a prepreg (hereinafter referred to as a glass epoxy resin) obtained by impregnating a thermosetting epoxy resin to a glass cloth (a non-woven fabric of glass fibers) is used as an insulator, and a conductive foil such as a copper foil is bonded thereto by hot press molding usually under such conditions as a pressure of from 10 to 40 kgf/cm.sup.2 at a temperature of from 170 to 230.degree. C. for from 30 to 120 minutes.
The glass epoxy resin is excellent in soldering heat resistance or chemical resistance and relatively inexpensive, but it has had problems that as it contains glass fibers, cracks are likely to be imparted when an impact of dropping, etc. is exerted, thus leading to conduction failure, and it takes a long time for curing the epoxy resin during the hot press molding and thus the productivity is poor.
Further, in recent years, in correspondence with miniaturization and reduction in weight of electronic appliances including notebook personal computers and portable telephones, high densification of wirings and miniaturization and reduction in weight of circuit boards have been required, and a study on a multi-layer board using a thermoplastic resin film as an insulating layer has been actively carried out for the purpose of satisfying the requirements.
When a thermoplastic resin film is used as an insulator for a printed circuit board, various merits can be expected. As compared with a conventional glass epoxy resin, miniaturization and reduction in weight of the circuit board can be realized, the impact resistance can be improved, and the molding time for the hot press molding can be shortened, which is advantageous from the viewpoint of the productivity. Inherently, the insulator for a printed circuit board is required to have soldering heat resistance from the process of its production, and if it is possible to use a heat resistant thermoplastic resin such as a polyether ketone resin or a polyimide resin, it can be expected to obtain a circuit which is excellent in the electrical properties at a high temperature and which is reliable in a high temperature atmosphere.
However, such a heat resistant thermoplastic resin has a high temperature for molding, and it will be necessary to use an adhesive such as an epoxy resin or to carry out hot press molding at a high temperature of at least 260.degree. C., for bonding with a conductive material or for forming a multilayer structure for the substrate, and it takes time to raise or lower the temperature, whereby the superiority of the thermoplastic resin will be lost in the productivity. Further, in the case of a crystalline resin, the bonding property can not be obtained unless it is heated to a temperature close to the melting point, and if the temperature exceeds the melting point, the resin turns to start flowing and undergoes flow deformation. On the other hand, the polyimide resin is excellent in heat resistance, chemical resistance, mechanical strength, etc., but the hygroscopicity is large, and the dimensional stability is inadequate, and the price of the starting material is very high.